1. Field of the Invention
The present invention pertains to a method for measuring signal travel times or ranges.
2. Description of the Related Art
Such a method is particularly suited to measuring the level in containers. From EP 0 780 665 A2, it is known to install in the container a waveguide, whose longitudinal direction runs in the direction that the level changes in the container, and to excite a high-frequency, electromagnetic signal in the waveguide. The impedance of the waveguide is changed at an interface between two media, e.g., air and a fluid, located in the container. The signal pulse excited in the waveguide by the known method is reflected in the region of the impedance change of the waveguide, and the reflected signal is received as an echo signal at the input of the waveguide. The time interval between transmitting the transmit pulse and receiving the reflected pulse is used as the measure for the range between the input of the waveguide and the location of the impedance change at the waveguide. From this time interval, the level of the material in the container can be determined.
For devices that evaluate signal travel times for range measurement, it is known from DE 42 40 491 C2 to superimpose a reference pulse on an echo signal that is reflected by an object, whose distance is to be measured. Thus, the reference pulse at the beginning of the echo signal gives information about the time of transmission of the transmit signal, so that the distance range to the object can be determined by evaluating the time difference between the reference pulse and the reflected pulse of the echo signal.
To determine the time position of the reflected pulse in the echo signal, it is known to compare the time profile of the received echo signal with the second reference signal that was formed without objects in the measurement path or with an empty container. Points in the echo signal curve where the amplitude of the echo signal curve deviate from the amplitude of the reference signal curve point to pulses due to reflection at the object or a change in impedance in the waveguide.
In this way, problems can arise from different amplitude offsets of the second reference signal or echo signal, which makes the comparison of the two signals more difficult.
Thus, the goal of the present invention is to provide a method for range measurement, in particular for level measurement, which enables a precise measurement through comparison of an echo signal with a reference signal, and with which there are no problems in connection with different amplitude offsets.
This goal is solved by a method for range measurement by means of a signal transmitted in the direction of an object to be measured by means of a transmission direction, wherein an echo signal featuring a first reference pulse is received by a receiver device, and wherein the echo signal is compared with a second reference signal that features a second reference pulse for determining a signal travel time, and wherein for determining an amplitude offset between the echo signal and the second reference signal, a first comparison value of the echo signal and a second comparison value of the second reference signal are compared with each other, and these signals, in regard to time, are before the first and second reference pulses in the regions of the echo signal curve and the second reference signal curve, respectively, and in these regions, these curves exhibit a slope that is at least approximately zero.
The present invention uses the fact that the echo signal features a constant amplitude that is different by an amplitude offset from the zero amplitude value within a time interval before the transmission of the signal pulse, and thus, before the appearance of the first reference pulse. The same applies to the second reference signal, which is a stored echo signal that was formed under certain conditions, usually for an empty container or without objects to be measured in the measurement path. In the regions of constant signal profiles before the first and second reference pulses of the echo signal and the second reference signal, the method according to the invention determines the offset between the echo signal and the second reference signal in order to better compare the echo signal and the second reference signal after the addition or subtraction of this offset from one of the two signals, and in order to thus be able to determine the reflection time of the signal pulse at the object to be measured.
The reference pulse of the echo signal and the reference signal can be pulses that have been inserted into the particular signal, especially for travel time measurement, as time reference points, or they can be pulses that in each echo signal result in a given measurement path, e.g., due to reflection of a transmitted signal at a transition between a line and a waveguide.
Advantageous configurations are the object of the invention.
The method according to the invention with offset compensation is suitable for all methods with which range measurement is performed by means of transmitting an optical, acoustic, or electromagnetic signal, and with which the range is determined by evaluating the signal travel times between transmitting a signal pulse in the direction of the object to be measured and receiving a pulse reflected at the object.
The method is particularly suited to measuring the level in containers by means of wave-bound microwave signals. The impedance of a waveguide, in which the microwave is guided, changes at the point where there is a transition between two different media, or also between two equivalent media with different densities. At this location of the impedance change, a partial reflection of the microwave signal excited in the waveguide is produced with a frequency that can be up to 2 GHz. Range measurement by means of a guided and reflected microwave is also referred to as the TDR method (TDR=Time Domain Reflectometry).
According to an embodiment of the invention, for each determination of the offset, a least upper bound (i.e., a maximum for positive pulses and a minimum for negative pulses) of the amplitude of the first reference pulse and a least upper bound of the amplitude of the second reference pulse are determined, and each time an amplitude value of the echo. signal curve and of the second reference signal curve, which lie at a predetermined time period before the respective least upper bound, are used as first and second comparison values. The time period is chosen such that the echo signal curve and the signal curve of the second reference signal exhibit at least an approximately constant amplitude during this time period before the least upper bound of the first and second reference pulses.
Instead of the maximum of the first and second reference pulses, output points for the offset determination can also be amplitude values that are under each maximum and that each have an amplitude that is a predetermined percentage of the respective maximum.
The determination of the offset between the echo signal curve and the curve of the second reference signal is done by the method according to the invention preferably by means of a signal processor, a conventional microprocessor, or a conventional computer executing a suitable program. Here, both the echo signal and also the second reference signal are provided as a sequence of detected values, wherein the frequency for generating the detected values from the time-continuous signals is greater than the maximum frequency of the two signals to be compared.
According to another embodiment of the method according to the invention, for the echo signal and the second reference signal for determining the first and second comparison values starting from the maximum or from the amplitude value that is in a fixed ratio to the maximum, adjacent amplitude values are compared with each other. Here, if regions of the signal curve are reached in which adjacent amplitude values, or adjacent detected values, are no longer different, then these amplitude values are used as comparison values.
In order to reduce the effect of unavoidably small oscillations of the echo signal and the second reference signal in the constant sections before the first and second reference pulses, several first and second comparison values are calculated, and their average value is used for determining the offset.
If necessary, in order to be able to eliminate the existing time offset between the echo signal and the second reference signal, according to another embodiment of the invention, the time position of the maximums of the first and second reference pulses are compared with each other.
A further object of the invention is a device for performing the method according to the invention.